During transmission of an information signal from a transmitter to a receiver in a communications system, the information signal typically modulates a carrier signal. The information signal may modulate the carrier signal using a wide variety of methods, such as amplitude, phase, or frequency modulation.
In an amplitude modulated (AM) stereo system, the amplitude of the carrier signal is typically modulated by the information signal such that a substantial amount of information may be transmitted in a relatively small band of frequencies. As well, stereo information associated with the transmitted signal may also be transmitted within the frequency band. Several systems for transmission and reception of AM stereo information have been developed through industry use. Each system implements a method for providing two audio channels within a predetermined band of frequencies with high quality stereo sound and very little interference. However, one of the standards, an AM stereo system which uses quadrature amplitude modulation, is used most often and is, therefore, a de facto industry standard.
An industry standard AM stereo system licensed by Motorola, Inc., under the trademark "C-QUAM" is referred to as a Compatible Quadrature Amplitude Modulation stereo system. The "C-QUAM" stereo system typically provides stereophonic information using amplitude modulation for a main information signal, and a quadrature type of phase modulation for a stereo information signal. Quadrature phase modulation is used to separate a composite of a left channel (L) and a right channel (R) of the stereo information signal, and a difference between the left and the right channels, by a phase angle of 90 degrees for transmission. A signal broadcast using the C-QUAM stereo system must then be separated into the composite of and the difference between the left channel and the right channel of the stereo information signal at a receiver.
In a "C-QUAM" stereo receiver, stereophonic components are typically extracted from a broadcast signal using standard analog circuits. The broadcast signal is converted to a pure quadrature information signal, and a quadrature demodulator is then used to extract both the composite and difference of the left and the right channels of the broadcast signal. If the broadcast signal has only the composite of the left and the right channels of the broadcast signal, the broadcast signal is monaural, or has no stereo components. The stereo components are transmitted as the difference between the left and the right channels of the broadcast signal.
Before the broadcast signal is input to the quadrature demodulator, the signal must be converted to an original transmitted quadrature signal which contains phase modulation components. This is accomplished by gain modulating the broadcast signal. To convert the broadcast signal to a base band signal, the broadcast signal must be demodulated with both an envelope detector and a sideband detector. The envelope detector demodulates the broadcast signal to provide a composite signal of the left and right channels of the broadcast signal. Similarly, the sideband detector demodulates the gain modulated broadcast signal to provide a difference signal indicating a difference between the left and right channels of the broadcast signal. The signals provided by both the envelope detector and the in-phase component of the sideband detector are then compared and the resultant error signal gain modulates the inputs of the sideband detector. Each of the composite and difference signals is then provided to a logic circuit referred to as a "matrix." The matrix processes each of the composite and difference signals to output a separate left and right channel signal. For further information on the operation of a "C-QUAM" encoder and receiver, refer to "Introduction to the Motorola "C-QUAM" AM Stereo System" published by Motorola, Inc. in 1985.
Although an analog solution adequately demodulates the base band signal and subsequently separates the base band signal into a left and a right stereo signal, the signal quality of the resulting left and right stereo signals is limited by the nature of the analog solution. For example, during the operation performed by the matrix, each of the composite and difference signals must have phase and amplitudes which are perfectly balanced. If the signals do not have balanced phases and amplitudes, the left and right channels of the base band signal are mixed and the resulting sound is distorted. With precise design and implementation, an analog solution may adequately balance both the phase and the amplitude of each of the composite and difference signals such that distortion is not readily noticeable. However, such precision is difficult to achieve.
Additionally, analog circuitry typically approximates a demodulation function. Therefore, the sound provided by an analog version of the C-QUAM receiver must be carefully monitored and processed to provide an audio sound which simulates the sound originally transmitted. As well, because the analog solution requires several components which are discrete, noise is produced during demodulation and during the transmission of information between each of the components. Additionally, in typical analog implementations of C-QUAM receivers, gain modulation is performed on an audio signal at the input of the C-QUAM receiver. Because the audio signal is typically sampled at a high frequency at the input, the precision and accuracy of the gain modulation operation may be limited by the speed with which the operation must be performed. Phase error and/or frequency error introduced during the demodulation may also result in increased distortion in the sound output by the analog version of the C-QUAM stereo system.
Therefore, a need exists for an AM stereo receiver which demodulates a broadcast signal to produce a high quality stereo signal. The stereo receiver should also not add any phase error or amplitude components which might distort the broadcast signal. Additionally, the AM stereo receiver should not introduce any extraneous noise which would further degrade the quality of the stereo sound. As well, the AM stereo receiver should also provide the stereo signal in a timely and economical manner.